Current systems for the production of electricity from geothermal energy rely on the heat in the earth's crust to vaporize water or another liquid; the vapor is then used in a turbine to generate electricity. The geothermal heat is generally brought to the surface via wells that tap into reservoirs of steam or brine that circulate at depths in the crust sufficient to collect a substantial amount of heat. See U.S. Pat. No. 3,786,858 (1974), for example.
However, modern steam turbines operate most efficiently at very high temperatures, substantially higher than the temperatures achieved in the steam or brine reservoirs generally used to produce geothermal electricity. The heat that is present at depths within the earth that are attainable (for practical purposes) is not sufficiently concentrated. Geothermally-powered steam turbines are therefore less efficient. They are also somewhat limited in operation by the fact that the heat removed from the earth cannot be stored for later use; it must be used immediately or lost.
In addition, the brine or steam that is brought to the surface loses a significant amount of its heat (generally 25% to 30%) as it comes to the surface. Brine or steam from geothermal reservoirs is generally accompanied by hydrogen sulfide and other undesirable gases, which must be captured before they escape into the atmosphere. Because the temperature of the brine or steam is relatively low, a large amount must be transported to the surface, and so large-diameter wells, which are expensive to drill, are required. Moreover, the brine and steam that is brought to the surface is often highly mineralized and corrosive. If it is used directly in a turbine, the turbine must be modified to withstand these conditions, which further deceases the efficiency of the system. In the alternative, the brine or steam may be used both directly and to boil another fluid through a heat exchanger in a binary generating system. These alternatives also lose some efficiency through the heat exchanger.
Another problem that can be caused by the minerals in the brine or steam is scaling in the wells, which can build up over time and must be removed. The brine presents problems of disposal after it has been used, unless it is reinjected into the reservoir, which requires expensive pumping and may contaminate the reservoir. Even if the brine is reinjected, some of the salts may drop out of solution as the brine is cooled prior to reinjection; these salts must be disposed of, and they may be radioactive or otherwise hazardous.
The most significant limitation, however, is that there are very few reservoirs that are both large enough and hot enough to make geothermal exploitation an economical prospect, and so the conventional method for geothermal production of electricity is very limited in application.
Research is currently being conducted into the possibility of drilling into hot, dry rock ("HDR") and injecting water to create a geothermal reservoir which can then be tapped to generate electricity; however, such systems face many of the same problems as conventional geothermal systems, and are more expensive. Prior art HDR systems require two wells to be bored, an injection well for injecting the water to create a reservoir, and a separate production well for continuously bringing the steam to the surface. Employing only one well for injecting water and retrieving steam would not be efficient since either too much energy would be lost when the injected water passes the rising steam, or the steam would be retrieved only intermittently so that energy would not be supplied to the generator on a continuous basis.
The injection of water into the rock requires an amount of energy that represents a significant fraction of the energy that the system can produce, thus lowering the efficiency of the system. Also, a certain percentage of the water that is injected is lost into fractures in the rock, and is not returned up the production well. The greater the amount of pressure that is used to drive the water from the injection well to the production well, the more water is lost, because higher pressure at the injection well causes the cracks to dilate there, as does the colder water, which causes the rock to contract. The dilation is needed at the production well, where it accelerates the release of the energy in the rock. Tests have shown that short-term shutting-in of the production well, by increasing dilation there, improves overall production from the well. Although subterranean reservoirs of magma achieve even higher temperatures than hot, dry rock or reservoirs of steam or brine, current geothermal systems are not capable of tapping directly into magma.
The predominant technology used for the generation of electricity is the combustion of hydrocarbons and the conversion of the resultant heat to electricity. Up until the last decade, most electricity was generated by the combustion of coal to produce steam. Recently, approximately half of all new electric generating capacity has taken the form of combustion turbines burning oil or natural gas and using the power to create electricity through a direct link to a generator. In a system using a "combined cycle," the heat from the combustion turbine exhaust is used to create steam, which then generates additional electricity in a steam turbine. However, each of the foregoing combustion processes releases substantial amounts of nitrogen oxides, creating air pollution and the potential for acid rain. They also produce carbon dioxide, thus contributing to global warming. If coal or oil is used as the fuel, sulfur dioxide is also released into the atmosphere, which may produce additional acid rain, and particulates may be released as well. The combustion of coal also produces ash, which must be disposed of properly. Moreover, these processes all consume limited natural resources. Also, a combustion turbine uses a significant amount of the energy it creates to compress the air that it takes in, in order to sustain its operation.
Other technologies used to produce electricity include nuclear, hydroelectric, solar and wind generation. Nuclear generation is expensive and presents serious issues of disposal and contamination. Hydroelectric, solar and wind generation face timing and geographic limits in terms of the areas in which they are effective and may require extensive collection systems or otherwise involve environmental impacts. Also, solar and wind generation are significantly more expensive than conventional technology.
Much of the electricity currently produced is generated by condensing steam turbines; fuel is combusted and the exhaust is released into the atmosphere, while the heat produces superheated steam. The steam passes through a steam turbine generator, generating electricity, at the end of which the steam is condensed. The drop in pressure due to condensation at the outlet end of the turbine permits the turbine to turn more freely, but the overall process is still less than forty percent efficient, in part due to the need to exchange the combustion heat into steam energy. Also, a significant amount of energy is lost through the exhaust of the combustion process.
A steadily increasing portion of new generating capacity installed in recent years is in the form of combustion turbines. Combustion turbines use the energy released from combustion to turn the shaft on a turbine, which then turns an electrical generator. The turbine requires a large volume of air for the combustion, which requires filtering and, often, heating or cooling, and introduces dirt into the turbine. The exhaust is released into the atmosphere, and it carries a significant amount of energy as well as pollution with it. In addition, a combustion turbine uses a significant amount of energy to compress the inlet air, only 16% of which is oxygen used in the combustion process.
Only recently have combustion turbines achieved efficiencies approximating 40% while thus operating in "simple cycle." Efficiencies approximating 50% can be achieved by combustion turbines operating in "combined cycle," in which the heat of the exhaust from the combustion turbine is exchanged into steam energy, which is then used to operate a steam turbine generator. This steam is not, however, as superheated as the steam that is ordinarily used in steam turbine generators and consequently the steam cycle of a combined cycle system is less efficient than a simple steam turbine.
The steam turbine and the combustion turbine (whether simple cycle or combined cycle) each cause pollution due to the release of products and byproducts of combustion into the atmosphere. They each lose efficiency because they release as exhaust a significant amount of the energy from the combustion. The steam generator and the combined cycle combustion turbine generator each lose efficiency due to the exchange of heat into steam pressure.